The DC bias will appear between the filament and ground.

While making changes you might want to implement this service update from Heathkit if it hasn't already been done, it was released for the SB-221 but given the near identical nature of the SB-220/221 outside of 10 meter facilities I expect it also applies to your unit IF you plan to later wire it for 240 volts which is the preferred primary feed:

April 26, 1985
SB-221 Bulletin No:
2 KW Linear Amplifier SB-221-14

T2 Overheats And Fails When Operating On 220 Volt Line

When the linear amplifier is wired for 220 volt AC line operation, and
the contacts on SW2 [PN 61-45] or a high voltage transformer winding
opens, excessive current will flow through the primary of low voltage
transformer T2 [PN 54-238],
causing it to fail. To prevent this failure, the black-green lead and
the black-yellow lead of T2 are lifted and connected together with a wire
nut.

To do this, refer to the drawing at the right and remove the black-green
lead at lug 3 of terminal strip AE [the lead from T2 at grommet AK] and
the black-yellow lead at lug 2 of SW2. Connect these wires together with
a wire nut [PN 432-199].

Make this wiring change on units you receive for service which are wired
for 220 volt AC operation. Let the customer know that this wiring change
has been made. If he wants to operate on 120 volts AC, he'll have to
reconnect these wires as shown in the assembly manual.

Rodger WQ9E

Thanks for the update Roger. I almost think it would be better to remove the extra primary wires from SW2 entirely, hardwired so that the full primary windings(CW tune mode) are always used, setting the hv to 3200 volts. max. Getting 10% more hv is hardly worth it.
I'm still not sure how to measure the grid bias. The 5.1v zener is isolated from ground by 1 ohm. But notice on the diagram I posted above for the new switch setting for the bias on the relay. The diagram shows the zener at circuit board ground(the little arrow for CB ground). Is this deliberate ?

Decided to take voltage reading with the tubes in sockets. With an alligator clip lead to one of the filaments,
I read about +2.5 volts dc. Not the 5.1v I am supposed to get. Not sure what this means.
Roger, and any others, what is your take on removing the complex network on the grids and just strapping them to ground as others have suggested. It seems to make sense since the Zener is providing the bias and not some dc voltage applied to the grids.

Was the bias measured with the amp keyed? What was the quiescent plate current? If you had plate current, you should have seen 5.1 volts DC across the Zener. Don't measure from either filament lead; the voltage reading is across the Zener.

From what I see, if you measure at a filament, you will have an additive 2.5 VAC voltage that is effectively in series with the DC bias voltage produced by the Zener. Does that make sense?

The AC filament voltage would cancel at the true center tap, which is where the Zener is placed.

As an aside, there were a few Dentron amps that actually had the AC filament voltage superimposed on the cathode biasing! That has been discussed in various forums... but it was not a problem with the Heath models.

Pete

Phil,

The idea of the network for the grids instead of directly grounding them is it provides measurably better IMD performance. Drake, Kenwood, and Swan use the same circuit and it has nothing to do with normal operating bias so you could directly ground them if you wish but my SB-220, all three versions of the Drake amp, and Swan Mark II are working fine with the original circuit and no voodoo additions. Bill Orr suggested this circuit in his design notes and handbooks and I believe this is why the manufacturers picked up this circuit although I don't think it showed up in the Eimac data sheets for this 3- family of zero bias tubes.

I looked at the modification that directly grounds the Zener and I am not seeing how that will work with the SB-220 grid metering circuit since the metering is across R3 which is shunted across the multimeter to read grid current. I wouldn't want to run the amp without the ability to monitor grid current.

It seems a lot of SB-220 modification articles showed up about the time that QST was becoming significantly less rigorous in vetting technical articles...

Rodger WQ9E

Roger-did you mean to say ?

I haven't seen any mods that ground the zener.

Phil,

I was referring to your December 02 post in this thread where you inquired about the location of R27 and you posted a modification schematic showing one end of the Zener directly grounded.

Rodger WQ9E